Ultrahigh Vacuum Process For The Deposition Of Nanotubes And Nanowires

ABSTRACT

A system and method A method of growing an elongate nanoelement from a growth surface includes: 
     (a) cleaning a growth surface on a base element; 
     (b) providing an ultrahigh vacuum reaction environment over the cleaned growth surface; 
     (c) generating a reactive gas of an atomic material to be used in forming the nanoelement; 
     (d) projecting a stream of the reactive gas at the growth surface within the reactive environment while maintaining a vacuum of at most 1×10 −4  Pascal; 
     (e) growing the elongate nanoelement from the growth surface within the environment while maintaining the pressure of step c); 
     (f) after a desired length of nanoelement is attained within the environment, stopping direction of reactive gas into the environment; and 
     (g) returning the environment to an ultrahigh vacuum condition.

RELATED APPLICATION DATA

This Application claims priority from U.S. Provisional PatentApplication Ser. No. 60/964,498, filed Aug. 13, 2007.

GOVERNMENT RIGHTS

This invention was made with government support under DE-FC52-05NA26999awarded by the Department of Energy. The government has certain rightsin the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nanotechnology, nanotubes and nanowiresand processes for the manufacture of nanosize technology.

2. Background of the Art

Nanotubes and nanowires of various materials have become the subjects ofintense, global research efforts in recent years. Fabrication of NTs andNWs made from carbon, nitrides and oxides, metals (e.g., Published USPatent Application 20060289351) and mixtures thereof (e.g., Published USApplication 20070057415) have been reported. The interesting combinationof electrical and mechanical properties of these NT and NW structureshas raised possibilities of revolutionizing fields ranging fromcomputing, optics, field emitter devices, sensors, electrodes, solarcells, high strength composites, hydrogen storage and many otherapplications.

To date, the most widely used process used to grow NT and NW is aChemical Vapor Deposition (CVD) process. In CVD process, a substrate,often predeposited with catalytic particles of transitional metals, isheated up to high temperature in presence of high pressure (up to 20Torr) of feedstocks (such as CO or a hydrocarbon in case of growingcarbon based NT and NW). One type of CVD process relies solely onheating of the substrate to promote catalytic breakdown of the feedstockat or around the particles and subsequent formation of NTs or NWs at theparticles. This type of CVD process is known as thermal CVD. In anothertype of CVD process, a strong electric field is applied during thegrowth process. Introduction of the electrical field generates plasma ofthe feedstock gas around the substrate. This type of CVD process iscalled Plasma Enhanced Chemical Vapor Deposition (PECVD). In PECVD,growth of NTs and NWs is facilitated by presence of more reactive gasescreated from dissociation and ionization of the feedstock gas within theplasma. Moreover, presence of electric field forces NTs and NWs to growin the direction of the electric field and thus achieving superioralignment of the NTs and NWs.

Although PECVD and thermal CVD have clearly demonstrated to be capableof producing NTs and NWs, there are some fundamental limitationsassociated with these processes. First, high-pressure requirements ofthe CVD processes do not allow growth surface of the substrate to beclean to atomic level and kept free of surface contaminants for anyappreciable time inside the reactor. This is true for both CVD growthtechniques, thus NTs and NWs are grown on top of a surface that is farfrom atomically clean. High-pressure conditions of the CVD reactors alsocontribute to increased contamination of exterior of NTs and NWs duringand after the growth. Second, the high-pressure requirements effectivelyeliminate usage of many, if not all, powerful in-situ deposition andanalysis tool that requires low-pressure environment to operate. Thus,in-situ processing and monitoring are difficult to carry out in the twoCVD processes. In PECVD, the chemical reactions at the substrate surfaceare difficult to control as the surface is directly exposed to theplasma consisting of many species of radicals, neutrals, ions andelectrons of different energies. Nanowires can also be provided withheterostructure by varying crystalline composition along its length astaught in U.S. Pat. No. 6,882,051.

All references cited herein are incorporated herein in their entirety.

SUMMARY OF THE INVENTION

The present invention enables a versatile Physical Vapor Deposition(PVD) process for the growth of nanowires (NWs) and nanotubes (NTs)constructed from a wide range of materials (metals, metal mixtures,carbons, nitrides, oxides etc.). The process uses molecular flows ofreactive gases under an ultra high vacuum (UHV) environment. The PVDprocess allows for growth of NTs and NWs to be carried out under ultraclean and low pressure environments without exposing the substrate tothe presence of high energy particles (ions, radicals, electrons etc.)of a plasma that can damage the substrate and any NTs and NWs growing ontop of the substrate. These features allow the PVD process to producecleaner nanotubes/nanowires with more controllable growth conditions.The PVD process offers the possibilities of developing recipes forgrowth of sophisticated nanowire structures (vertical P/N junctions,superlattices, etc.) by combining the nanotube/nanowire growth processwith a wide range of UHV deposition/functionalization techniques.

The new deposition process described in this invention addresses theabove weaknesses of the CVD processes. The new deposition process allowsdepositions of NTs and NWs to be carried out in an extremely cleanenvironment with the background pressure level that is at least fiveorders of magnitude less than a typical CVD process during all phases ofthe growth. Main advantages of this process are: (1) substrate surfacecan be cleaned to atomic level and preserved in this clean condition foran extended period, (2) substrate surface morphology (or surfacereconstruction) can be controlled and monitored prior to and duringgrowth, (3) fluxes of highly reactive gases with thermal energy (andthus low impact damages to the surface) can be precisely and rapidlycontrolled, (4) surface reactions at the substrate can be betteranalyzed and optimized since the surface is not affected by theprocesses involved in generating the reactive gases, (5) potential forlow temperature growth (below 500° C.) by utilizing high reactivity ofthe gases, (6) ability to instantaneously modify strength and directionof electrical field above the growth surface, (7) ability to carry outin-situ doping, post growth (of NTs) deposition or otherfunctionalizations, and, (8) the ability to use the entire arsenal ofthe latest surface treatment and analysis tools that can only operateunder UHV conditions.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic of a chamber for use in the practice of thepresent technology.

FIG. 2 shows a flow diagram for performing a non-limiting example of ananoelement manufacturing process in a pressure chamber useful in thepractice of the present technology.

DETAILED DESCRIPTION OF THE INVENTION

The absence of mass within a volume and the reduction of vapor orgaseous presence within a volume is the basis for pressure measurements.The degree of absence of mass is the basis for distinguishing degrees ofvacuum, which have been essentially standardized within the varioustechnical fields as:

Pressure range (See note below) Pascal Pascal (absolute mode) (absolutemode) ‘Degree of vacuum’ 1 × 10⁵ to 3 × 10³ 100 000 to 3 000     lowvacuum  3 × 10³ to 1 × 10⁻¹ 3 000 to 0.1     medium vacuum 1 × 10⁻¹ to 1× 10⁻⁴ 0.1 to 0.000 1 high vacuum 1 × 10⁻⁴ to 1 × 10⁻⁷ 0.000 1 to 0.000000 1 very high vacuum  1 × 10⁻⁷ to 1 × 10⁻¹⁰ 0.000 000 1 to 0.000 000000 1 ultra-high vacuum (UHV) <1 × 10⁻¹⁰ <0.000 000 000 1extreme-ultrahigh vacuum (EHV or X) Note: The two lists of pressureranges shown in the table above are numerically identical, the leftcolumn style being known as scientific notation. Atmospheric pressure isnominally 1 × 10⁵ Pa (100 000 Pa) so, for example, high vacuum covers arange that is somewhere between one millionth and one thousand millionththe value of nominal atmospheric pressure.

For purposes of the practice of the present invention, UHV shall be anypressure of ≦1×10⁻⁷ P, up to any commercially enabled level of pressure.The length of the nanofilaments, nanotubes and the like that can begrown has not been found to be limited except by the size of thechamber. Lengths of millimeters, centimeters and up to 100 cm or moreare theoretically possible based on early experimentation. Diameterchanges are controllable to a lesser degree based on the size (diameter)of a starting catalyst spot on the cleaned surface and/or other factorsthat determine an initial diameter size from which the filaments andtubes are grown.

The present technology comprises method and apparatus and resultingnanoelement products. The method may be described as a method of growingan elongate nanoelement (e.g., nanowire, nanotube, nanofilament) from agrowth surface. Growth surfaces may be any composition known in the artas useful for supporting growth of nanowires (NWs) or nanotubes (NTs)under prior art procedures, usually a dielectric material of highpurity, such as a silica surface. The process may be generally describedas:

(a) cleaning a growth surface (cleaning being done according to at leastconventional standards applied to surfaces in chip, circuit and/or boardmaking processes currently used);

(b) providing an ultrahigh vacuum reaction environment over the cleanedgrowth surface;

(c) generating a reactive gas of an atomic material (e.g., especiallyatomic materials such as carbon, metals, metalloids, and other materialsused for forming nanotubes of nanofilaments under Chemical VaporDeposition) to be used in forming the nanoelement;

(d) projecting a stream of the reactive gas at the growth surface withinthe reactive environment while maintaining a vacuum of at most 1×10 ⁻⁴Pascal, wherein at least 90% of any elevation in pressure results fromintroduction of the reactive gas into the environment;

(e) growing an elongate nanoelement from the growth surface within theenvironment while maintaining the pressure of step c);

(f) after a desired length of nanoelement is attained within theenvironment, stopping direction of reactive gas into the environment;and

(g) returning the environment to an ultrahigh vacuum condition.

Seeding of the initiation of the growth, which can strongly influencethe diameter of the growing nanoelement can be done by physicallyseeding the surface with a nanoparticle, providing a nanocatalyst spot(as is done in growing nanotubes) before step d), or can be accomplishedby the initial projection of materials in step d) above.

The new PVD process may be carried out in commercial chambers ormodified chambers that can support the ultrahigh vacuum and are fittedwith the reactive gas projector. For example, the process can be carriedout in a stainless steel vacuum chamber.

The vacuum chamber may contain strategically located flanges with UHVseals for mounting of a substrate holder, in-situ monitoring processingtools (such as mass spectrometer and RHEED gun, as shown in FIG. 1) andUHV compatible sources (such as effusion cells as shown in FIG. 1) forgenerating reactive gaseous species. All source flanges are oriented sothat sources mounted on these flanges may be or are directed toward thesubstrate. The inner wall of the chamber is lined with a cooling panelsuch as a liquid nitrogen (LN₂) cooled cryopanel. The chamber is pumpedto a base ultrahigh vacuum pressure of, for example, 2×10⁻¹⁰ Torr orless prior to introduction of reactive gases into the chamber. When thecryopanel is cooled with LN₂, the base pressure can reach down to 10⁻¹¹Torr.

The substrate manipulator should be capable of continuous azimuthalrotation around its axis to improve uniformity across the wafer. Theheater behind the sample is designed to allow heating of the wafer tovery high temperature (up to 1,000° C.) without impurity outgassingwhile achieving excellent temperature uniformity across the wafer(+/−1%@ 700° C. under UHV). The special heater element used should bechemically inert and is compatible to be used in presence of reactivegases including oxygen, these heater elements being commerciallyavailable for other CVD processes.

Reactive gases needed for growth of NTs and NWs are generated inside thechamber by two different types of the sources. These sources may be:

-   -   1. Effusion cells in which highly pure solid charges are        thermally heated to generate beams of molecular gases directed        toward the sample. Effusions cells can be operated up to        2,000° C. and are suitable for evaporating materials such as        Aluminum, Galium, Indium and Zinc. Beams of these gases are        needed during growth of nitride NTs/NWs and oxide NTs/NWs.    -   2. Radio Frequency (RF) plasma sources for generating reactive        oxygen species (for growing oxide NTs/NWs, that is nanotubes or        nanowires), nitrogen species (for growing nitride NTs/NWs) and        hydrocarbon species (for growing carbon NTs/NWs). The type of RF        plasma source utilized consists of a small cavity surrounded by        the radio frequency (RF) coil. The end plate of the cavity        contains an aperture plate with a collection of tiny apertures        through which the activated gas escapes the source and enters        the chamber.        Some UHV compatible in-situ monitoring tools that can be used to        facilitate optimization of NTs/NWs growth procedures include        Reflectance High Energy Electron Diffraction (RHEED) for        monitoring crystal structure of the growth surface prior to        growth, Auger Electron Spectroscopy (AES) for elemental        characterization of surface adsorbates before and after the        growth, Quadruple Mass Spectrometry (QMS) for analysis of        residual gases inside the chamber before, during and after the        growth.

A Kauffmann type broad beam ion source can be used for sputter cleaningof the wafer. A set of electrodes may be placed near the substrate forintroducing electric field around the substrate with the ability tochange orientation of the electric field during growth.

Step 1: Growth Surface Cleaning and Preparation

Since the new PVD process is carried out under UHV conditions, growthsurfaces of a wide range of substrates can be prepared, prior to thegrowth, to be completely free of surface contaminants with the outermostsurface atoms arranged in a specific orientation that is most favorablefor high quality NTs/NWs growth. Under UHV, surface contaminants can beremoved in three possible ways. First, the wafer can be heated up tovery high temperature for thermal desorption of adsorbates. Si surface,for example, can be made free of native oxides by heating the wafer to1,000° C. Second surface cleaning process is ion sputter cleaning In atypical ion sputter cleaning step, an inert gas ion (Ar⁺ in most cases)of about 500 eV in energy is used to bombard the substrate. Incident ionbeam physically removes surface contaminants present in the growthsurface. Third surface cleaning process is reactive surface cleaningReactive surface cleaning utilizes thermal energy hydrogen atoms oroxygen atoms to reactively clean the growth surface at low temperature.Reactive surface cleaning process is the preferred cleaning method forthose substrates that cannot tolerate high temperature processing and/orbe damaged by bombardment of ions with 500 eV incident energy. All ofthe above substrate cleaning methods, as well as other methods, can beutilized to clean the growth surface for the proposed UHV PVD growthprocess.

Once the growth surface has been cleaned, prevailing UHV condition ofthe vacuum chamber allows the growth surface to be maintained at thisatomically cleaned condition for an extend period of time. (At chamberbase pressure of 1×10⁻¹⁰ Torr, the expected time required to form 1monolayer (ML) of surface adsorbate is about 10,000 seconds.) Duringthis time period in which the growth surface remains clean, one canthermally anneal the substrate to (1) remove any surface damages causedby sputtering and make the growth surface atomically smooth, or (2)thermally rearrange outer most atoms of the growth surface to form aspecific surface reconstruction that is most favorable for growth ofNTs/NWs.

Prior to growth, confirmation of atomically clean growth surface can bemade by performing AES or another appropriate technique. Identificationof the specific surface reconstruction pattern can be observed withRHEED or another technique.

As the initial nucleation steps of NTs/NWs growth are stronglyinfluenced by surface properties of the substrate, the new PVD processwith the ability to prepare the growth surface with atomic levelprecision, offer much advantages over conventional thermal CVD andPECVD.

Step 2: Generation of Reactive Gases

Reactive gases needed for growth of NTs/NWs are generated by eitherthermally heating of effusion cells containing solid charges or runninggases through discharge zones of RF plasma sources. Pure O₂ and N₂ gasesmay be used to generate reactive species of oxygen and nitrogen. Amixture of hydrocarbon and Argon gases are used to generate reactivespecies of carbon containing molecules.

Fluxes of reactive gases generated by effusion cells are controlled byadjusting temperatures of the effusion cells. Fluxes of reactive gasesgenerated by RF plasma sources are adjusted by controlling flow rates ofthe gases to the sources. Composition (i.e., ratio of the metastableexcited molecular species and atomic species) of oxygen and nitrogenbeam can be adjusted by changing the RF power and gas flow used duringthe discharge.

Once the sources for the reactive gases are in full operational mode andstart producing beams of gases inside the chamber, the chamber pressurerises to about 10⁻⁵ Torr. At this working pressure level, mean free pathlength of the reactive gases are still longer than the distance betweenorifices of the sources and the substrate. The significance of this isthat each gaseous particle leaving the source reaches the substratewithout encountering collisions with background gases. Thus the gasparticles generated by each source form a beam that travels to thegrowth surface under molecular flow condition. Under this condition,each gas particle leaving a source maintains its energy state and traveldirection during its journey toward the growth surface.

The schemes for generating and transporting the reactive gases utilizedby the new PVD process allow for superior control of chemical reactionsat the growth surface. In the PVD process, plasma discharges needed togenerate reactive gases occur far away from the growth surface. Thus,chemical reactions at the growth surface that produces NTs and NWs arenot affected by the high energy reactions occurring in the plasma. Incontrast, in a PECVD process, the growth surface is surrounded by theplasma and chemical reactions at the surface are likely to be influencedby most, if not all, of the reactions occurring at the plasma.Additionally, in the new PVD process, properties (incident energy andmomentum, electronic and chemical state, etc.) of each reactive gasparticle leaving the source remain unchanged during its travel towardthe growth surface. Thus, one can, to large extent, select properties ofthe reactive gases hitting the surface by controlling the operatingparameters of the sources. This kind of control of incident gasparticles is not possible in the CVD and the PECVD processes developedto date for growing NTs and NWs.

Step 3: Growth of NTs and NWs

Once the substrate has been properly prepared and the sources are tunedto produce molecular beams of reactive gases, growth of NTs/NWs isstarted by simply opening the shutters in front of the sources and thusexposing the growth surface to incoming molecular beams of reactivegases. The growth can be terminated by simply interrupting the flows ofthe incident beams by closing the source shutters.

For the growth of carbon based NTs/NWs, a molecular beam of crackedhydrocarbon molecules from a RF plasma source is needed. For nitrideNTs/NWs, co-deposition of a group III beam (Ga, In, Al) from an effusioncell and/or activated nitrogen beam from a RF plasma source is needed.And for oxide NTs/NWs, co-deposition of activated oxygen beam from RFplasma source with a beam of element such as Zn, Ti and other materialare needed.

The presence of electrodes around the substrate can be used to createelectric field near the growth surface. As the effusion cells evaporateonly neutral molecular species, and the RF plasma source can be tuned toemit only neutral species of reactive gases, incident beams needed forgrowth of NTs/NWs are not affected by presence of the electric field.However, if the chemical reactions at or near the surface involves ionicspecies, introduction of electrical field could provide the necessaryguiding force to direct growth of NTs/NWs in the direction along theelectrical field. Taking this possibility one step further, one may beable to abruptly change growth direction of the NTs/NWs by changingorientation of the electrical field while NTs/NWs are growing. Usingthis idea, it may be possible to develop a recipe for growing NTs/NWswith specific shapes or contours.

Because the chamber allows co-deposition of many materials and flux ofeach source can be introduced or interrupted rapidly (typically lessthan 0.1 second), the new PVD system is capable of growing more complexnanoscale structures such as superlatticed NTs/NWs and doped NTs/NWs.For example, growth of GaN/InN superlattice NTs/NWs maybe achieved bycontinuously exposing the growth surface to nitrogen beam whilealternatingly exposing the surface to Ga and In beams. In-situ doping ofNTs/NWs may be achieved by exposing the growth surface to a beam ofdopant gas (generated from an effusion cell) while NTs/NWs are growing.

Step 4: Post Growth Treatments

Once NTs/NWs are successfully grown, these nanostructures can be furtherprocessed inside the vacuum chamber prior to removing the nanostructuresfrom the UHV environment. Some of the post growth treatments include:

-   -   Reactive surface cleaning: It has been reported that carbon        nanotube (CNT) are often covered with amorphous carbon deposits        on outer walls of the tube. This undesirable coating of        amorphous carbon may be reactively etched away by exposing CNTs        to a beam of atomic hydrogen generated by the RF plasma source.        Atomic hydrogen beam with thermal energy can be produced by        running the plasma source with H₂ gas. Since it is likely that        atomic hydrogen will preferentially attack amorphous carbon,        in-situ hydrogen cleaning is expected to do little damage to CNT        lattice during the limited time needed to etch away amorphous        carbon.    -   Low energy ion beam in range of 40 eV has been demonstrated to        be effective in implanting Fullerenes. At this energy, the        incident ions have enough energy to penetrate the carbon lattice        of Fullerenes without damaging it. Given the structural        similarity of Fullerenes with CNTs, it is expected that an ion        beam of the similar energy range can be an effective tool as        post growth implanting of CNTs. In the PVD chamber, this post        growth ion implanting can be carried out by simply irradiating        CNTs to a beam of low energy ions generated by an appropriate        ion source mounted to the chamber.

Additional Process Benefits

1. The process described herein can be viewed as relying upon molecularflow of cracked hydrocarbon species directed at the substrate. Thismeans that incident energy and electronic state of a cracked hydrocarbonspecies (as generated by the RF plasma source) will remain unchangedfrom the time they leave the source until they hit the surface. This isbecause the cracked hydrocarbons species do not collide with each otheror with back ground gases as until it hits the surface, so that it isthe species themselves that directly impact the surface and may attachor react thereto. There is no intermediate reaction with the majority(at least 50%, at least 70%, at least 85%, at least 95%) of the crackedhydrocarbon species before contact with the surface or other speciesattached or reacted to the surface. Thus, our process allows thepossibility of growing carbon nanotubes (CNT) using a beam of crackedhydrocarbon species that have certain specific distribution of theelectronic state and molecular species. For example, by using differentconductance apertures or different level or RF power to the source, wemay make the beam more rich in uncracked but metastably excited naturalhydrocarbon species or use another aperture plate or different gas flowof higher RF power to make the beam more rich in cracked hydrocarbonspecies. This ability to prepare specific types of beam composition canonly be done with the process and not with a UHV CVD process. Thus theprocess includes the ability to change, alter, and controllably vary andbuild layer content and region content of species on the surface. A plancan be implemented where a specific distribution of different speciesand concentration and volume and area of species is designed, and thatplan implemented by process control (usually by designed programming) toproduce continuous distributions of species, discontinuous (e.g.,patterned) distributions of specific or single species, patterns anddistributions of at least two different species or variations (partiallyreacted, different states, different crystallinity) of a single speciesor multiple species, graded thicknesses of multiple species (e.g.,beginning with 100% of one species at the surface and graduallytransitioning to a different concentration of that one species to thesurface which may be from 0.5% to 100% of a second species or multiplespecies), and continuous layers with two-dimensional (along the plane orparallel to the plane of the surface) variation in composition.

2. Because the presently described process relies on molecular flow ofthe reactive species, we can control the incident direction of the beam.This allows for in-situ masking and possibly even a combinatorialdeposition of CNT. In contrast, any kind of UHV chemical vapordeposition (CVD) process can not attempt these tasks as gas flow aroundthe substrate is more or less uniform. It is also possible to provideangular gradation of shape, thickness and content in the process byangular direction of the species.

3. Finally, In CVD process, the growth surface is not protected from anykind of plasma process employed to promote the growth. This is becausethe growth surface sits in middle of the plasma envelope. This meansthat the growth surface morphology and catalytic particles on thesurface are bombarded by high energy ions and other charged particlespresent in the plasma. Thus, the exact condition of the growth surfacecan not be maintained or controlled during growth. In contrast, thepresently described process allows the total separation of the growthsurface from the plasma needed to generate activated or crackedhydrocarbon species. This is because cracking or activation of thehydrocarbon species is done inside the discharge tube of the RF plasmasource that is located in significant distance away from the growthsurface.

1.-17. (canceled)
 18. A system for growing an elongate nanoelement froma growth surface comprising: a support for a base element having agrowth surface; a chamber with gas flow controls to support an ultrahighvacuum within the chamber, wherein the chamber supports an ultrahighvacuum reaction environment of less than 10⁻⁷ Torr over the growthsurface within the chamber; and a source of directed reactive gas of anatomic material, wherein the reactive gas comprises a crackedhydrocarbon species.
 19. The system of claim 18, wherein a stream of thereactive gas is projected at the growth surface within the reactionenvironment while maintaining a vacuum of at most 1×10⁻⁴ Pascal whilegrowing an elongate nanoelement.
 20. The system of claim 18, wherein thechamber comprises an inner wall wherein the inner wall is lined with acooling panel.
 21. The system of claim 20, wherein the cooling panel isa liquid nitrogen cooled cryopanel.
 22. The system of claim 18, whereinthe chamber further comprises flanges for mounting a substrate holder,in-situ monitoring tools and compatible sources for generating reactivegaseous species.
 23. The system of claim 22, wherein the substrateholder is capable of continuous azimuthal rotation around its axis. 24.The system of claim 22, wherein the in-situ monitoring tools areselected from the group consisting of Reflectance High Energy ElectronDiffraction, Auger Electron Spectroscopy and Quadruple MassSpectrometry.
 25. The system of claim 22, wherein the compatible sourcescan be effusion cells or radio frequency plasma sources.
 26. The systemof claim 20, further comprising a heater element.